Particulate matter (hereinafter referred to as PM), nitrogen oxide (hereinafter referred to as NOx), etc. which may induce a risk of affecting the environment are contained in exhaust gas discharged from an internal combustion engine such as a diesel engine or the like. An exhaust gas purification system (Selective Catalytic Reduction (SCR) system) for injecting and mixing reducing agent such as urea solution, hydro carbon (HC) or the like into exhaust gas and selectively reducing and purifying NOx with catalyst is known as an exhaust gas purification system used to purify NOx.
An injection type for directly spraying liquid reducing agent into an exhaust gas passage through a reducing agent injection valve is known as an embodiment of a reducing agent supply device in the SCR system.
Here, in the urea solution out of the reducing agent materials described above, urea is easily crystallized in a predetermined temperature area, which causes clogging of the reducing agent injection valve. Foreign materials such as PM, etc. are frequently contained in exhaust gas irrespective of the kind of the reducing agent, and these foreign materials may invade into a nozzle or a reducing agent injection valve and induce seizing or the like, so that clogging occurs in the reducing agent injection valve.
Therefore, there has been proposed a method of determining a clogging of a reducing agent injection valve in an injection type reducing agent supply device by detecting the pressure of a supply passage for reducing agent. More specifically, there has been disclosed a method of determining by a pressure sensor 324 disposed in a reducing agent supply passage 321 whether a pressure reduction amount due to injection of reducing agent is normal or not and then determining clogging of a reducing agent adding valve (reducing agent injection valve) 320 when the pressure reduction amount is not normal as shown in FIG. 9 in an exhaust gas purification system having an injection type reducing agent supply device. As a specific example of the method, there has been disclosed a method of referring to a map which is obtained in advance by experimentally determining the correlation between the pressure reduction amount and the injection amount under a normal state, and comparing a normal value of the pressure reduction amount obtained from this map with a value obtained from the pressure sensor to determine abnormality of the pressure reduction amount (see Patent Document 1).
patent Document 1: JP-A-2006-274856 (Scope of Claim for patent paragraph [0016])